The present invention relates to a method for fabricating a semiconductor device in which a logic circuit and a nonvolatile memory are incorporated.
In recent years, to achieve multifunctionality and easy debugging, attention has been given to embedded flash memories in each of which a logic circuit and a nonvolatile memory are incorporated. A logic circuit incorporated in such an embedded flash memory needs to have the same function as that obtained when only the logic circuit is provided.
To fabricate an embedded flash memory, however, a process for fabricating a nonvolatile memory needs to be added to a usual process for fabricating a logic circuit. This additional process for fabricating a nonvolatile memory causes characteristics of the logic circuit to differ from those obtained when only the logic circuit is fabricated.
Accordingly, if a logic circuit optimized for a usual fabrication process for a logic circuit is fabricated by a fabrication process for an embedded flash memory, characteristics of the logic circuit are caused to change, and desirable characteristics are not obtained. If the logic circuit is optimized for the fabrication process for an embedded flash memory, desirable characteristics will not be obtained as well in a future process for fabricating a semiconductor device from which a nonvolatile memory has been removed.
The characteristics of the logic circuit change mainly because an insulating film buried in a trench isolation is etched during the fabrication of the nonvolatile memory. When the buried insulating film is etched, a reverse narrow channel effect, i.e., the phenomenon that an electric field from a gate electrode is concentrated at the end of the trench isolation to reduce the threshold voltage of a transistor, becomes conspicuous.
The buried insulating film is etched deeply especially in a process using hydrofluoric acid or a process using a mixed solution (an Ammonia Hydroxide-Peroxide Mixture: APM solution) which includes ammonia water and hydrogen peroxide and is generally called ammonia acid. Examples of the process using hydrofluoric acid include the process of removing a natural oxide film. Examples of the process using an APM solution include RCA cleaning typically used to clean a substrate or to remove a photoresist. These processes are repeatedly performed not only in the process of fabricating a logic circuit but also in the process of fabricating a nonvolatile memory, so that the addition of the process for fabricating a nonvolatile memory causes extra etching of the buried insulating film in the fabrication process for an embedded flash memory.
In an embedded flash memory, a high-voltage transistor for controlling write or erase operation is needed in a logic circuit. To form the high-voltage transistor, implantations for forming a well and for controlling a threshold voltage using photoresists for forming the high-voltage transistor as masks are required after a trench isolation has been formed, resulting in extra etching of a buried insulating film during removal of the resists.
As described above, the formation of an embedded flash memory involves extra etching of a buried insulating film to a greater extent than in the case of forming only a logic circuit. Consequently, characteristics of the logic circuit change.
As a means for suppressing etching of the buried insulating film in the trench isolation, the following method is proposed in Japanese Unexamined Patent Publication (Kokai) No. 6-151876. FIGS. 19A through 19C are cross-sectional views showing respective process steps of a conventional method for fabricating a semiconductor memory device in order.
First, as shown in FIG. 19A, an isolation film 10 and a tunnel film 14 are formed on a silicon substrate 2. Then, as shown in FIG. 19B, a first polysilicon film 16 is formed within a memory cell region M1, and then an ONO film 18 as a stack of silicon dioxide (SiO2), silicon nitride (SiN) and silicon dioxide (SiO2) is formed over the entire surface of the substrate including a peripheral transistor region T1.
Next, as shown in FIG. 19C, the ONO film 18 is removed except for parts located on the memory cell region M1 and the isolation film 10. Thereafter, a second polysilicon film 20 is formed and then gate electrodes are formed, thereby forming transistors in the memory cell region M1 and the peripheral transistor region T1.
With the structure described above, the ONO film 18 formed on the isolation film 10 serves as a protective film for preventing the isolation film 10 from being etched. Accordingly, during removal of a natural oxide film and RCA cleaning which are repeatedly performed to form the transistors in the memory cell region M1 and the peripheral transistor region T1, the isolation film 10 is not etched so that the film thickness is not reduced. As a result, deterioration of electrical characteristics of transistors caused by reduction of the film thickness of the isolation film 10 is prevented.
However, the conventional method for fabricating a semiconductor memory device has the problem that the protective film, i.e., the ONO film, remaining on the isolation region causes charge to be captured in the protective film on the isolation region, so that the property of isolating the transistors deteriorates. This is because of the following reason. A silicon nitride film tends to capture charge, and thus charge is easily trapped in the silicon nitride film when an electrical stress is applied in, for example, a process for forming transistors. The trapped charge is held, resulting in that a parasitic transistor is formed.
As to a structure in which the protective film remains only on the isolation region, part of the protective film inevitably overlaps an active region in an actual fabrication process. This overlapping part of the protective film serves as a gate insulating film, causing the problem that a mask misalignment or a variation in size greatly change characteristics of peripheral transistors in a logic circuit.
Optimization of the logic circuit with the protective film left causes another problem that a protective film is needed so as to keep characteristics of the logic circuit from changing or that another logic circuit needs to be optimized when a product from which a nonvolatile memory has been removed is fabricated after termination of debugging.